Elsevier

Ecological Engineering

Volume 34, Issue 2, 2 September 2008, Pages 137-146
Ecological Engineering

Constructed mangrove wetland as secondary treatment system for municipal wastewater

https://doi.org/10.1016/j.ecoleng.2008.07.010Get rights and content

Abstract

Intermittent subsurface flow mangrove microcosms were constructed to investigate their capabilities in treating primary settled municipal wastewater collected from a local sewage treatment work in Hong Kong SAR and the effect of hydraulic retention time (HRT). The study was carried out in a greenhouse and without any tidal flushing or tidal cycle, with half of the tanks planted with Kandelia candel and half without any plants. The removal percentages of dissolved organic carbon (DOC), ammonia-N, inorganic-N and total Kjeldahl nitrogen in the planted systems were 70.43–76.38%, 76.16–91.83%, 47.89–63.37% and 75.15–79.06%, respectively, significantly higher than in the unplanted system during the 6-month treatment period. More than 97% ortho-phosphate and 86.65–91.83% total phosphorus were removed by the planted microcosms. The HRT of 10 days had better removal than that of 5 days, and the best performance was obtained in the planted microcosms with 10 days of retention time. During the 6-month experimental period, the concentrations of all forms of nitrogen in the treated effluent were within the standards for effluents discharged into Group B inland waters and coastal zone with open waters. In terms of phosphorus, the effluents met the standards for effluents discharged into Group A inland waters. These results suggest that it is feasible to use the constructed mangrove wetland without tidal flushing as a secondary treatment process for municipal wastewater.

Introduction

Wetland treatment systems have been considered as an alternative to conventional treatment methods, especially for small communities (Soukup et al., 1994, Kivaisi, 2001, Solano et al., 2003), because of their low treatment cost and low maintenance, especially in suburban or rural areas without any large-scale sewage treatment facilities. In the past few decades, constructed wetlands have been applied in treating municipal (Gumbricht, 1992, Kaseva, 2004, Chung et al., 2008), industrial (Mays, 2001, Jacob, 2004, Gottschall et al., 2007), agricultural and livestock wastewater (Comín et al., 1997, Knight et al., 2000). The feasibility of employing constructed wetlands as a secondary treatment system is attractive because it saves land areas; however, the subject still needs more research. Most of the existing constructed wetlands use perennial plants such as common reeds, cattails and bulrush, which are strongly recommended to be harvested periodically (Hammer, 1989, Cronk, 1995, Gopal, 1999). Frequent harvesting would lead to poor plant growth and increase the cost of manpower (Cronk, 1995, Gopal, 1999). Hammer (1989) therefore suggested that other species of perennial plants should be used to avoid annual plantings.

Mangrove swamps commonly found along tropical and subtropical coastlines, usually between 35°N and 35°S latitude, are some of the most productive wetlands in the world (Tam and Wong, 2000, Al-Sayed et al., 2005). In previous decades, mangroves in many countries have been seriously damaged by human activities, and ecological restoration of mangroves through mangrove planting has been commonly used to reverse such losses in the last decade worldwide (Lewis, 2005). Because of their high productivity, mangrove plants have a large demand for nutrients and many mangrove habitats struggle with a nutrient deficiency problem (Li, 1997, Alongi et al., 2005). The beneficial effects of nutrient addition on growth of mangroves have been reported (Clough et al., 1983). Mangrove plants that have colonized the inter-tidal habitat are also well known for their ability to withstand various environmental stresses, including high salinity, waterlogging, alternating aerobic and anaerobic conditions and unstable substratum (Clough et al., 1983). Chiu and Chou (1993) reported that Kandelia candel could transfer oxygen from its aerial parts to its roots, and a portion of oxygen would be leaked from the roots into the adjacent soil, producing a thin aerobic zone surrounding around the plant roots. The very extensive root systems of mangrove plants thus create a significant aerobic zone in the rhizosphere for oxidation (Chen et al., 1995). In addition, the mangrove soil and roots were found to harbor diverse groups of microorganisms which played essential roles in nutrient transformation (Al-Sayed et al., 2005). These unique functions of mangrove ecosystems create a suitable environment for removing and transforming pollutants in wastewater. Some previous studies have demonstrated that mangrove wetland systems made a significant contribution to the removal of nutrients and organic matter from wastewater (Sansanayuth et al., 1996, Wong et al., 1997, Chu et al., 1998, Tam et al., in press). However, there are problems with using mangrove wetlands for nutrient removal. One of them is the need of frequent tidal flushing, which makes the use of mangroves less practical. It will be more cost-effective if the mangrove wetland treatment system can be operated without regular tidal cycles. Our recent research has demonstrated that mangrove plants can grow well in fresh water without tidal flushing (Tam, 2004). The present study aims to determine the performance and efficiency of constructed subsurface-flow mangrove wetlands, without tidal flushing, as a secondary treatment system. The role of mangrove plants K. candel in treating primary settled municipal wastewater under two hydraulic retention times (5- and 10-day HRTs) will also be investigated. Among eight true mangrove plant species found in Hong Kong, K. candel is the most widely distributed and common species. It can be found in all mangrove swamps, while for the two genera in the same family as Kandelia, Bruguiera is present in 28 out of 44 mangrove swamps and Rhizophora is absent in Hong Kong (Tam and Wong, 2000).

Section snippets

Experimental set-up

The mangrove wetland treatment microcosms consisted of 12 PVC tanks. Each microcosm had the dimensions of 0.67 m (L) × 0.54 m (W) × 0.38 m (D). Individual microcosms were divided into three zones, inlet (0.12 m × 0.54 m × 0.15 m), treatment (0.45 m × 0.54 m × 0.15 m) and outlet (0.10 m × 0.54 m × 0.15 m), and filled with different media (Fig. 1). In inlet and outlet zones, the media consisted of gravel (around 2.27–2.91 cm in diameter), while the treatment zone was filled with soil collected from Sai Keng, a typical

Dissolved organic carbon

The mean concentration in the influent was 72.79 mg L−1 with temporal fluctuations (Fig. 2). The effluent concentrations in the planted microcosms were lower than the unplanted ones at both retention times. The DOC removal was generally higher in treatments under longer retention time; for instance, in planted microcosms, 70.43% and 76.38% removal were obtained at 5- and 10-day HRT, respectively (Table 2). According to the results of two-way ANCOVA (Table 3), both HRTs and plants significantly

Discussion

The removal percentages in the planted systems were 70.43–76.38% for DOC, 76.16–91.83% for ammonia-N, 47.89–63.37% for inorganic-N, 75.15–79.06% for total Kjeldahl nitrogen, >97% for ortho-phosphate and 86.65–91.83% for total phosphorus. These removal efficiencies were comparable to and even higher than that reported by previous studies of commonly used perennial wetland plants (Table 4). The quality of most treated effluent was able to meet the discharged standards set the Hong Kong Government

Conclusions

From the 6-month wastewater treatment study, it is clear that the mangrove microcosm with intermittent subsurface horizontal flow is effective in removing organic matter, nitrogen and phosphorus from primary settled municipal wastewater. The treated effluents, in terms of all forms of nitrogen and inorganic phosphate, were able to meet the effluent discharge standards for inland waters set by the Hong Kong Environmental Protection Department. The planted mangroves had higher treatment

Acknowledgement

The work described in this paper was supported by the Areas of Excellence Scheme established under the University Grants Committee of the Hong Kong SAR, China (Project No. AoE/P-04/2004).

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